Technical Abstract: Disease is the result of a detrimental relationship between a host and either another organism or its environment. Weedy plants can become diseased, and there has been particular interest in using microbes to control them. This chapter emphasizes disease caused by microbial organisms, describing the concept of disease, how disease is caused, epidemiology and disease, the kinds of organisms cause disease, ways that organisms can be used for weed control, and how disease has been used to control invasive plants. Examples are provided to illustrate successes. Particular emphasis is given to interactions between pathogens or between pathogens and insects. There are two general approaches to the use of pathogens, the classical approach, involving introduction of an organism into a new area, and the bioherbicidal approach that involves application of the pathogen at high concentrations much like the way we apply chemical herbicides. Consideration is given to new or "out of the box" ideas, including the bioherbicide approach and organisms that interact with each other for control of rangeland weeds.